Systems for exchanging text messages or making a voice call via a public communication network such as the Internet or mobile communication network have become popular.
For example, in a system generally referred to as an instant messaging system, once a member logs onto a predetermined server device through a terminal device, he/she can exchange messages or make a voice call in real time with another member who is currently logged onto the system among the members whom he/she has allowed to exchange the message with.
Some of such instant messaging systems even enable a video call that uses not just voice but also video or enable the exchange of data files.
For example, Patent Document 1 discloses a system as follows: when, while a member exchanges text messages with another member, a third member makes a call to the member and a conversation is started, the anxiety or the discomfort of the member with whom the text messages have been exchanged can be relieved by sending notification to the member that the message cannot be replied because the member is now on a call.